Methods and apparatuses for apportioning fuel to an internal combustion engine are known from the prior art, in which the fuel is delivered pressurized into a fuel accumulator device by means of a pump. Injectors, which are assigned to the respective cylinders of the internal combustion engine, extract fuel from the fuel accumulator device and apportion it at predeterminable points in time in a predeterminable quantity to the respective cylinder of the internal combustion engine. The apportioned quantity of fuel can be adjusted here to different operating states by way of the opening time of the injector, the number of individual injections and the fuel pressure in the fuel accumulator device.
The fuel pressure in the fuel accumulator device can be regulated to a desired value, which is dependent on the operating state, by means of a control device. A pressure sensor is provided here in order to detect the actual value. If the measured actual value is lower than the provided desired value, the actual value can be increased by actuating the fuel pump.
If the actual value is higher than the target desired value, a controllable return valve allows for the decrease in pressure.
If the fuel pressure in the fuel accumulator device is incorrectly set as a result of the control loop, the fuel quantity fed to the combustion process changes in an unwanted fashion. This mostly results in unwanted behavior of the internal combustion engine and of the motor vehicle equipped therewith, like for instance loss of power, increase of power, irregular engine operation, poor emission values, high consumption or suchlike. Since the internal combustion engine in a motor vehicle is operated with frequently changing operating conditions, it is not possible for an on-board diagnosis system, which is integrated in the engine control device, to determine the precise cause of fault of the injection system. Possible causes of fault are for instance the return valve, the fuel pump, the injectors or the pressure sensor. This frequently results in undamaged components being replaced. Such a replacement is always associated with time and effort and costs, without the motor vehicle subsequently functioning properly again. Customer satisfaction is herewith negatively influenced.